Thromboembolic disease is a major cause of morbidity and mortality in the developed world. Because platelets play a critical role in the activation and propagation of thrombosis, they represent important targets for new anti-thrombotic drugs. A potent inhibitor of human platelet function has been isolated from the bloodfeeding hookworm, Ancylostoma caninum. This novel protein is the first naturally occurring compound shown to block the binding of platelet integrins GPIIb/IIIa and GPIa/IIa, the cell surface receptors for fibrinogen and collagen, respectively. The objective of this Phase I STTR grant application is to purify milligram quantities of active recombinant hookworm platelet inhibitor (rHPI) suitable for testing in vivo and in vitro. The rHPI will be characterized using whole platelet assays of aggregation and adhesion. In addition, quantitative experiments will measure the ability of rHPI to block the interaction of collagen and fibrinogen with their respective integrin receptors. Competition experiments with various peptides will delineate HPI binding sites and further characterize its molecular mechanism of action. Lastly, truncated variants of rHPI will be produced in order to determine the smallest protein domain with preserved antiplatelet activity. The long-term objective is to develop rHPI as a therapeutic agent for the treatment of thrombotic disorders in humans.